“That quake did not occur on the Hikurangi margin but was around 24km west and is known as an intra-plate earthquake.”
Dr Cave said there was still some discussion about the sense of movement of the Te Araroa earthquake but the USGS (United States Geological Survey) had indicated that it was largely a “‘strike slip” where the movement was horizontal.
“This is consistent with its location, west of the Hikurangi Margin. As it did generate a small tsunami, there's likely to have been some degree of normal (vertical) movement as well as horizontal displacement,” he said.
The Hikurangi Margin is an active subduction zone extending off the east coast of the North Island, where the Pacific and Australian plates collide.
“The 6.1 earthquake associated with the Te Araroa quake was located further east, close to the main Hikurangi margin fault, but at this stage is still considered to be an intra-plate earthquake.”
A few hours after the 7.1 shake on Friday morning, an earthquake of magnitude 7.4 hit 966km north of East Cape in the Kermadecs, followed by a larger 8.1 closer to this at 8.28am.
“As was the case with the Te Araroa earthquake, the initial locations were provisional and have since been amended,” Dr Cave said.
“Both of the Kermadec quakes were ‘thrust faults', with the 8.1 quake occurring on or very close to the plate margin, which is the northern extension of the Hikurangi Margin.
“Both quakes are indicative of a subduction-type movement where rocks on the Pacific Plate slide underneath the rocks of East Cape,” he said.
The Te Araroa 7.1 and Kermadec 8.1 quakes generated small tsunami.
“The Te Araroa earthquake showed as a one-metre oscillation on the Te Araroa tide gauge while the Kermadec quake produced a smaller 30cm-50cm oscillation at the gauge.
“These were expressed largely as very active shore waves surging back and forth.
“At Tokomaru Bay a bore was videoed by Claudia Maaka (a still shot from it was on the front page of Saturday's Herald).
“While the video looked dramatic it did not result in waves above the normal high tide mark at the shore.
“At Hicks Bay, some deeper water seaweeds were deposited at the high tide mark.”
Dr Cave said two tsunami occurring in rapid succession was very unusual.
“The Tokomaru bore was probably more obvious as a result of the wave being pushed up as it became confined in the bay. This did not occur at Te Araroa as it is more open to the ocean.”
Unusual wave surges were still occurring at Te Araroa at midday Saturday, he said.
“That highlights that the effect of tsunami can last for more than 24 hours after the event.
“The position of these quakes offshore of East Cape makes accurate determination of their size and precise location difficult due to the distance from the onshore seismograph network.
“The magnitudes of the Kermadec quakes has been determined by USGS and GeoNet advises that it's best to use the USGS magnitudes as the GeoNet system under-reported their magnitudes,” Dr Cave said.
“GNS scientists will work on analysis of these events in the coming week and data will be updated, and we will update the Tairawhiti community as soon as that information comes to hand.”
